Perimeter Calculator
Free perimeter calculator for squares, rectangles, triangles and circles. Enter the side lengths and get the exact perimeter or circumference in seconds.
About this tool
The perimeter is the total distance around a shape - the fence around a garden, the frame around a picture, the edging of a lawn. For circles it has a special name: the circumference.
Pick your shape, enter its measurements, and this perimeter calculator gives you the exact answer in seconds.
How to use
Choose square, rectangle, triangle or circle, then fill in only the fields for that shape. For a triangle enter all three sides - the tool checks they can actually form a triangle.
Press Calculate Perimeter and the result appears instantly, in the same units you measured with.
The math behind it
Square: 4 x side. Rectangle: 2 x (length + width). Triangle: a + b + c. Circle: 2 x pi x radius.
A triangle is only valid when each side is shorter than the other two combined - the calculator verifies this for you.
Frequently asked questions
What is the difference between perimeter and area?
Is circumference the same as perimeter?
Why did it say my triangle is invalid?
Related tools
💬 Comments
Have a question about this page, or an idea to make it better? Share it below — your feedback helps everyone who uses this site.